Collars or necks are required on workpieces such as, for instance, plates or other prefabricated components for various purposes as, for example, guide members, liners, stiffening members, or threaded members. The collar required for these purposes is produced on the workpiece, in many cases by means of a forming method known as collar forming or neck forming.
When an exact form of the collar, for instance, its length or its diameter, is important, first of all the workpiece can be punched, for example. Subsequently, the previously produced hole can be widened by means of a partially conical, pointed or rounded collar punch, wherein a collar is generally formed perpendicularly to the workpiece. The degree of expansion can in this case be made dependent on the collar length to be achieved and/or on the expansion size to be achieved.
In the event that the collar length and/or the edges of the collar do not have to satisfy any particular quality demands, it is alternatively a known practice to shape the collar without previously punching the workpiece, for instance by means of flow drilling or extrusion. In the case of flow drilling, it is possible to increase the length of the collar on account of local massive forming and of the associated material store and incremental forming, although the shape accuracy and precision of neck forming sometimes suffer as a result.
However, when the collar is produced without local massive forming, it should be noted that the widening of material represents a considerable load for the particular material, since in particular the edge regions are stretched to an extreme extent. If the stretching capacity of the material is exceeded in the process, either marginal stretches with reduced possible applications or material failure occur locally, and the workpiece ruptures or tears in the region of the collar.
Thus, the hole expansion capability is dependent not only on the initial hole dimension or the hole dimension to be achieved, but also on the material itself, this setting limits to the achievable collar size and/or collar length. Thus, for instance, soft, deep-drawable steels have a good hole expansion capability. In a corresponding manner, the hole expansion capability of high-strength steels is lower, with the result that in particular the collar lengths that are achievable with these materials can be greatly limited.
However, in order nevertheless to increase the achievable collar length, it is a known practice from U.S. Pat. No. 1,613,961, for example, to initially form a hole or an opening in a planar metal sheet to avoid cracks in the production of necks. Subsequently, the metal sheet is inserted into a tool and the opening in the punched, planar sheet is widened by means of a conical punch and a collar is shaped. In that case, there should be sufficient material around the hole to shape the collar with the desired length.
German Patent Publication No. DE102006029124A1 likewise discloses a method and a device for forming necks from metal sheets using a punch and a counterholder, wherein the shaping height of the neck forming collar is intended to be achieved reliably. In that case, a planar metal sheet provided with a hole is inserted into a device and a collar is shaped there by means of a punch and a die. An improvement in reliability is intended to be achieved in that a counterholder is formed in an articulated manner, in order to be able to fit closely against the metal sheet better.
However, the above approaches do not allow the shaping of collars, in particular with regard to the length thereof, to a satisfactory extent and/or implementation of such approaches is comparatively complicated, for example, as a result of the use of hydroforming. Therefore, a need persists for increased collar lengths even with high-strength and other metals, or high-strength steels, but without the need to resort to relatively complicated methods such as hydroforming.
It is conceivable initially to provide a deep-drawing method, a subsequent punching operation, and subsequent expanding in a combination tool. However, it is a problem that sharp cutting edges that are required for punching can cause severe notching of the material during deep drawing when use is made for example of a combination tool. Notching can reduce the quality of the subsequent collar wall and/or does not allow the required drawing depth to be achieved.
It would likewise be conceivable in theory to provide a rounded base of the drawing punch. However, this approach would result in less material being able to flow out of the flange region and into the wall region, and this would reduce the drawing depth and collar depth.
In the particular case of an unfavorable drawing ratio, such as when the original workpiece diameter or the flange region is large compared to the diameter of the deep-drawn region, for example, there continues to be the need to increase the achievable collar length without the base region thinning out and base cracks occurring. In this regard, it would be conceivable for additional material reserves to be created in the workpiece in order to draw more material into the wall region during deep drawing, and this could indeed increase the collar length.
Yet one problem with the prior approaches is that the collar does not have a homogeneous workpiece thickness distribution as a result of the combined forming operations, because the collar has thinned-out regions at various points, for example in the region of the material reserves in the region of the punching and in the region of the expanded punching.
However, because the collars serve as, for example, liners for receiving movable axles or shafts, as described initially, they have reduced bearing surfaces as a result of the inhomogeneous sheet thickness distribution and accordingly cannot function correctly. Rather, clearance-type conditions would be required. Likewise, the inhomogeneity of the thickness distribution of the workpiece would result in different collar lengths arising, which in turn requires downstream trimming operations.